The Effect of Rain-Induced Stratification and Surface Roughness on Aquarius SSS Retrieval
[13-Nov-2014] Tang, W., Yueh, S., Fore, A., and Hayashi, A.
Presented at the 2014 Aquarius/SAC-D Science Team Meeting
The objective of this study is to develop a rain roughness correction to reduce the uncertainty of Aquarius SSS retrieved under rainy conditions. The calibration reference chosen by the Aquarius project calibration/validation team is the SSS from HYCOM (SSSHYCOM), with Geophysical Model Function (GMF) built using rain-free data. Under rainy conditions, surface salinity stratification associated with rain freshwater inputs may cause large discrepancy in salinity measured by Aquarius at 1-2 cm (the penetration depth of L-band radiometer) and SSSHYCOM a few meters below the surface. The sensitivity of model function to rain accumulation is tested by comparing radiometer GMF built using data under currently rain free condition (within 1 hour) with those built using only data free of rain in the past 24 hours, with rain history extracted from NOAA CMORPH data at 30-minute intervals. A method is proposed to use the rain impact model (RIM) [Asher et al., 2014; Santos-Garcia et al., 2014] to adjust SSSHYCOM to reflect near surface stratification caused by freshwater inputs accumulated from rain events occurred over the past 24 hours prior Aquarius overpassing. It is hypothesized that when calibrated with RIM adjusted SSSHYCOM (named SSSRIMHYCOM), the residuals, i.e. the difference between measured and model predicted TB under rainy conditions, represent the rain-induced roughness. The JET PROPULSION LABORATORY Combined Active Passive (CAP) algorithm is used to retrieve SSS in parallel with (SSSCAP_RC) or without (SSSCAP) rain roughness correction. ?SSS (SSSCAP_RC-SSSCAP), which highly correlate with rain pattern as expected, reaches about 0.2 PSU in wet areas such as ITCZ. Validation is performed with salinity measured at 50 cm by drifters (S50cm) and at 1m and 10m by mooring buoys. The difference between satellite retrieval (both SSSCAP and SSSCAP_RC) and in situ salinity measurements increases with rain rate, as well as the difference in depth. With rain-induced roughness accounted for, SSSCAP_RC likely represents the surface stratification effect in terms of the negative slope of SSSCAP_RC-Sin_situ(depth) vs. rain rate, which decreases with depth (towards the surface).